The present invention relates to an apparatus for detecting dew-point of a gas stream or changes in the dew-point properties of a gas.
A variety of devices (for example, as shown in U.S. Pat. No. 3,812,596, issued May 28, 1974 to Wilmanns) based upon the principle of detecting the presence of dew on a cooled surface, for example a mirror, by means of light reflection techniques, are currently available for the determination of the water dew-point of gas streams, particularly humid air streams. However, their performance is not always as reliable and accurate as might be desired. Humid air is essentially a two-component mixture consisting of a single condensable component in, for all practical purposes, an incondensable carrier. Dew-point in such a mixture is therefore easily defined. However, many gas streams, such as those found in the onshore and offshore gas industry, and in gas processing and industrial plants, are often complex mixtures for which the dew-point is less readily defined. Such a mixture can be regarded as a series of condensable fractions, and dew-point is then defined as that temperature, at fixed pressure (or vice versa), when measurable dew can be detected. Further decrease in temperature will increase the amount of dew formed as more of the heavier fractions first condense. It has been found that quantities of heavier fractions present in small, but still analytically significant, quantities have a profound influence on the dew-point of such a mixture. No reliable automatic dew-point meter or monitor that can respond sensitively and accurately to the formation of the first significant condensation of such heavier components, which define the dew-point, is believed to be available.
In order to obtain an accurate indication of the dew-point it is necessary to meet predetermined requirements as to temperature and pressure and it will be necessary to present a gas sample to be investigated under controlled conditions to the detector.
The most successful of current devices for complex mixtures of gases are those based on the visual observation of dew on a cooled plane-mirror surface. Their sensitivity is poor, however, and the observation and interpretation of visual dew formation is subjective and susceptible to operator bias or mis-reading.
Initial work using this principle, but with electronic detection of the change in light reflectance, showed that the signal obtained was noisy, transient and unreliable. Condensed water is relatively easy to detect as it condenses in a drop-wise manner, but complex mixtures of gases condense with much lower contact angles and quickly form a film on the surface, thus restoring reflection.